1. Field of the Invention
The present invention relates to an optical disc apparatus, and more particularly to an adjustment to the inclination (tilt) of an optical disc relative to an optical axis of laser light.
2. Description of the Related Art
A recordable optical disc apparatus such as a CD-R/RW drive or a DVD-R/RW drive has a problem that stability of data recording/reproducing cannot be maintained when an optical disc tilts relative to an optical axis of laser light (in a tilt state). Accordingly, a technology for providing a tilt sensor to the optical disc to detect the amount of tilt and adjusting the inclination of an optical pickup according to the detected amount of tilt has been proposed.
A block diagram of an optical disc apparatus with a tilt sensor is outlined in FIG. 10. An optical disc 10 is driven by a spindle motor 12 to be rotated. An optical pickup 14 is placed so as to be opposed to the optical disc 10. Laser light irradiated from the optical pickup 14 toward the optical disc 10 is reflected from the optical disc 10 and converted to an electric signal at a photo detector provided in the optical pickup 14. The signal from the optical pickup 14 is provided to an RF signal processor in which a tracking error signal TE and focus error signal FE are generated and provided to a tracking controller 50 for controlling tracking of the optical pickup 14 and focus controller 52 for controlling focusing of the optical pickup 14, respectively.
A tilt sensor 54, which is independent from the optical pickup 14, is installed opposed to the optical disc 10 to detect the amount of tilt. Information on the amount of tilt detected by the tilt sensor 54 is passed to a tilt driver 56 which adjusts the inclination of the optical pickup 14 so as to cause the amount of tilt detected by the tilt sensor 54 to approach zero. As a result, the optical axis of laser light is controlled to be almost perpendicular to the optical disc 10.
FIG. 11 shows the structure of the tilt sensor 54 of FIG. 10. The tilt sensor 54 includes a laser diode (LD) 54a, a two-segmented photo detector 54b and a differential amplifier 54c. Laser light irradiated from the LD54a is reflected from the optical disc 10 and then enters the two-segmented photo detector 54b comprising two detectors A and B where laser light is converted to an electrical signal. The LD54a and the two-segmented photo detector 54b are placed in such a manner that the same amount of reflected laser light enters both two detectors A and B in the two-segmented photo detector 54b when the amount of tilt of the optical disc 10 is zero. Accordingly, when the amount of tilt of the optical disc 10 is zero, the quantities output from the detectors A and B in the two-segmented photo detector 54b are equal to each other, with a result that there is no output from the differential amplifier 54c, in other words no tilt error signal is output.
On the other hand, when the optical disc 10 is inclined, the balance of incident quantities between the detectors A and B will be lost causing the quantities output from the detectors A and B to be unequal. As a result, the tilt error signal is output from the differential amplifier 54c. Because a signal level of the tilt error signal is proportional to the amount of tilt of the optical disc 10, the tilt driver 56 can adjust the inclination of the optical pickup 14 in accordance with the signal level of the tilt error signal.
However, the above-described structure in which the tilt sensor 54 is installed independently from the optical pickup 14 to detect the amount of tilt has a problem that the tilt error signal from the tilt sensor 54 does not precisely reflect the inclined amount of the optical disc 10 relative to the optical pickup 14. This problem is not resolved by placing the tilt sensor 54 in the vicinity of the optical pickup 14. Further, there is another problem that the overall cost of the apparatus is increased by providing a separate tilt sensor 54.
Another structure for performing tilt control based on the amount of returned laser light which is irradiated from an optical pickup toward an optical disc and then reflected from the optical disc, without providing a separate tilt sensor is disclosed in Japanese Patent Laid Open Publication 2001-184690. More specifically, the tilt is here controlled so as to keep the maximum value among the amounts of reflected light obtained when laser light at reproducing power, erasing power, and recording power is irradiated to an optical disc at a smaller level (i.e. to make the amount of reflected light obtained when recording power is applied smaller). By irradiation of laser light at recording power, marks are formed on the optical disc. However, the occurrence of tilt reduces the power of the laser light incident to the optical disc surface and causes failure of forming marks due to insufficient power, with a result that reflectance is not decreased. The amount of tilt, therefore, is controlled by executing an adjustment for minimizing the amount of reflected laser light.
As described above, when laser light is irradiated onto the optical disc at an angle, desired marks are not formed due to the reduced power of the laser at the optical disc surface. When this happens, however, the amount of reflected light transmitted to the photo detector is also reduced by the angled irradiation of laser light onto the optical disc. The increase of reflectance due to mark forming failure for lack of power caused by the tilt of incident laser light and the decrease of the amount of reflected light itself caused by the tilt are canceled out each other. Accordingly, such a structure for minimizing the amount of reflected laser light may cause inaccurate control of the amount of tilt.